The present invention relates to generally a power steering system for an automotive vehicle and more particularly a power-steering pump including a flow-control valve which may attain very stable flow rate attenuation characteristic, that is, the ability of reducing the flow rate of the hydraulic liquid under pressure to be delivered from a pump mechanism to a power steering system with increase in rotational speed of an engine, whereby the hydraulic liquid under pressure may be delivered at an optimum flow rate into the power steering system depending upon the operating conditions or speed of the engine.
In the power steering systems, the required hydraulic liquid under pressure is provided, in general, by a pump drivingly coupled to an engine so that the discharge from the power-steering pump changes as a function of the rotational speed of the engine. The slower the speed of the vehicle, the higher the steering load or resistance encountered becomes, and the steering resistance becomes the maximum during parking when the engine is idling or when the rotational speed of the engine is lowest. Therefore the power steering system is provided with a flowcontrol valve so that (1) the hydraulic liquid under pressure flows from the pump to the steering system at the maximum flow rate during parking in order to permit the easy steering and (2) the difference between the flow rate controlled by the flow-control valve and the flow rate when the rotational speed of the engine is high may be minimized. However, in practice the steering resistance during high speeds is considerably reduced than during parking so that there is a considerable difference between the flow rates during high speeds and parking. As a result, the safety in steering is adversely affected, and the undesirable heat evolution occurs.
To overcome these problems, the flow-control valves must have such flow-rate attenuation characteristic that the flow rate of the hydraulic liquid under pressure to be delivered to the power steering system may be reduced with decrease in steering resistance resulting from the increase in speed. The power-steering pump based upon the above principle is disclosed in U.S. Pat. No. 3,349,714, granted to Emile P. Grenier, Oct. 31, 1967. This pump utilizes the function of the control spool of a flow-control valve that it maintains the flow rate constant depending upon the increase or decrease in discharge from an oil pump. That is, a metering rod which is attached to the control spool is extended through a detecting orifice with a clearance there between so that the opening area of the orifice may be varied by the displacement of the metering rod. Therefore the gain of the flow-control valve is varied depending upon the rotational speed of the engine. However, the flow rate controllability of this flow-control valve is adversely affected by the dimensional errors of the metering rod and the misalignment of the metering rod with the orifice during assembly. Therefore they must be machined, finished and assembled with a higher degree of accuracy with the resultant increase in number of fabrication and assembly steps. In addition, there is another defect that since the flow-control valve with the metering rod is assembled as a unitary construction with the power-steering pump, it cannot be removed from the pump without disassembling the pump itself. As a result, the flow rate attenuation characteristic cannot be readily changed depending upon the types of the automotive vehicles and power steering systems.